Unveiling the Origins of Sulfur in Comets

A team of astrophysicists led by Duncan V. Mifsud from the University of Kent, UK, has conducted an experimental study to investigate the origin of sulfur-bearing molecules in cometary ices. They discovered that sulfur ions from the solar wind play a role in shaping the sulfur chemistry of Kuiper Belt objects—icy bodies beyond Neptune that include comets and dwarf planets. However, this contribution is relatively minor compared to the sulfur these objects inherited from the earliest stages of the solar system formation.

Sulfur-bearing molecules, such as sulfur dioxide (SO₂), are important components of cometary ices. They have been observed in both the bulk ice and the coma (the glowing envelope of gas surrounding a comet's nucleus) of the well-known comet 67P/Churyumov-Gerasimenko. The origins of these molecules in comets remain debated. Did they form over the comet’s lifetime, get implanted via the solar wind, or come directly from the material that formed during the birth of the solar system?

Figure 1: A top-view schematic diagram of the ICA experimental setup, showing how ions hit the ice surfaces at a 36° angle while an infrared beam passes straight through the ice to analyze the results.

To explore these possibilities, researchers conducted experiments using the Ice Chamber for Astrophysics-Astrochemistry (ICA). This experimental setup simulates space-like conditions to test how sulfur ions from solar winds interact with comet-like ice compositions. In their experiments, they studied four types of ices:

1. Pure oxygen (O₂) ice at 20 K (-423°F/-253°C).

2. Carbon monoxide (CO) ice at 20 K.

3. Carbon dioxide (CO₂) ice at 20 K.

4. Carbon dioxide (CO₂) ice at 70 K (-333°F/-203°C), to assess how higher temperatures affect sulfur chemistry.

The ICA setup, illustrated in Figure 1, includes a vacuum chamber to maintain extremely cold temperatures, an ion beam system to bombard the ice samples with sulfur ions, and an infrared spectrometer. This spectrometer uses light to detect chemical changes in the ice by measuring how it absorbs or reflects specific wavelengths.

Comet ices are composed of a mix of materials, primarily H2O, O2, CO2, and CO. Previous work examined the effects of solar wind on a pure H2O ice, so this experiment seeks to test the other major components. In the experiment, sulfur-bearing molecules were detected in the O₂ ice and the coldest CO₂ ice, but not in the CO ice or the warmer CO₂ ice. These findings highlight how factors such as ice composition and temperature significantly influence sulfur chemistry in cometary ices.

In summary, sulfur ions from solar winds do contribute to the formation of sulfur-bearing molecules in cometary ices, but their impact is relatively small compared to the sulfur that comets inherited from pre-solar environments. These results provide valuable insights into the chemical processes shaping Kuiper Belt objects and the early solar system.

This article made use of the following publication:

Mifsud, Duncan V. et al. 2024, Icarus, 411